The present invention relates to an omnidirectional antenna and in particular to an antenna which is omnidirectional in the bearing plane and whose radiating diagram or pattern in the elevation plane may exhibit a predetermined directivity.
Such antennas are used, inter alia, in the field of electromagnetic detection and also in that of telecommunications.
Omnidirectional antennas are known and particular mention may be made of those termed "discone" antennas, which consist chiefly of two conical reflectors whose apices are turned towards one another and which are fed through these very apices.
A discone antenna which is omnidirectional in bearing has a diagram whose directivity in the elevation plane is related to the size of the radiating aperture in this plane, and if a radiation diagram having small side lobes is required in the elevation plane, it is necessary for the pattern of illumination to show a small phase error. This is explained by the fact that, if the radiation diagram in elevation is to be very narrow, it is necessary for the radiating aperture to be very large, which results in phase errors in the field distribution across the aperture and causes the side lobes to increase again.
Thus, the error in phase may be reduced provided the radiating aperture is reduced by making the angle of the cones of the antenna smaller, but in this case the reduction in phase error is only achieved at the cost of an increase in the length of the discone. The result is a substantial increase in the physical size of the discone and this has a disadvantageous effect on the weight and bulk of the antenna.